In the lower placental mammals, the cerebral hemispheres leave the
proper upper and posterior face of the cerebellum completely visible,
when the brain is viewed from above; but, in the higher forms, the
hinder part of each hemisphere, separated only by the tentorium (p.
281) from the anterior face of the cerebellum, inclines backwards and
downwards, and grows out, as the so-called "posterior lobe," so as at
length to overlap and hide the cerebellum. In all Mammals, each
cerebral hemisphere contains a cavity which is termed the 'ventricle,'
and as this ventricle is prolonged, on the one hand, forwards, and on
the other downwards, into the substance of the hemisphere, it is said
to have two horns or 'cornua, an 'anterior cornu,' and a 'descending
cornu.' When the posterior lobe is well developed, a third prolongation
of the ventricular cavity extends into it, and is called the "posterior
cornu."

In the lower and smaller forms of placental Mammals the surface of the
cerebral hemispheres is either smooth or evenly rounded, or exhibits a
very few grooves, which are technically termed 'sulci,'separating
ridges or 'convolutions' of the substance of the brain; and the smaller
species of all orders tend to a similar smoothness of brain. But, in
the higher orders, and especially the larger members of these orders,
the grooves, or sulci, become extremely numerous, and the intermediate
convolutions proportionately more complicated in their meanderings,
until, in the Elephant, the Porpoise, the higher Apes, and Man, the
cerebral surface appears a perfect labyrinth of tortuous foldings.

Where a posterior lobe exists and presents its customary cavity--the
posterior cornu--it commonly happens that a particular sulcus appears
upon the inner and under surface of the lobe, parallel with and beneath